1. Field of the Invention
The present invention relates to an optomagnetic recording/reproducing apparatus and, more particularly, to an improvement in a magnetic circuit arranged in an optomagnetic recording/reproducing apparatus.
2. Description of the relating art
An optomagnetic recording/reproducing device has been recently developed as an information recording/reproducing device capable of optically recording, reproducing, or erasing information. A conventional optomagnetic recording/reproducing apparatus, for example, comprises an information recording medium composed of a magnetic thin film magnetized perpendicularly to its surface. In such an apparatus, predetermined information is recorded or erased such that the directions of magnetic moments in the information recording medium are inverted by the effects of a light beam and an external magnetic field. Information stored in the information recording medium is read out by utilizing the photo-electromagnetic effect which is known as the Faraday effect and the Kerr effect, i.e., that the angle of polarization defined by light incident onto the information recording medium and light emitted therefrom changes in accordance with stored information. For example, in the information recording mode, a light beam is locally incident onto a predetermined area of a recording surface. When the temperature of the area is raised over the Curie point, a magnetic field larger than the coercive force of a magnetic material of the local area and smaller than the coercive force of other areas is externally applied in order to invert the magnetic moments in the local area, thereby recording information. In the information erase mode, a light beam is incident onto a predetermined area of the recording medium. When the temperature of the area is raised over the Curie point, an external magnetic field larger than the coercive force of the magnetic material is uniformly applied from a predetermined direction in order to uniformly align the directions of the magnetic moments, thereby erasing information. The optomagnetic recording/reproducing apparatus comprises the information recording medium, a light source for emitting a light beam on the information recording medium, a detector for receiving the light beam from the information recording medium, and the like. In addition, a magnetic field must be externally applied to a recording surface irradiated with a light beam, as described above. For this reason, a bias magnetic field generator is also arranged in the device. Such a bias magnetic field generator is disclosed in Japanese Patent Disclosure (Kokai) No. 58-200407.
Various improvements in the bias magnetic field generator have been introduced in order to effectively generate a magnetic field. For example, FIG. 1A shows a generator, in which a coil is simply wound around a light pickup member, and the resultant pickup assembly is located above a surface of recording medium 3 so as to oppose it. FIG. 1B shows a generator in which magnetic material 4 is arranged to concentrate the magnetic field generated by coil 2. FIG. 1C shows a generator in which permanent magnet 5 extends along a recording area of recording medium 3.
FIG. 2 is a schematic view of a structure of an optomagnetic recording/reproducing apparatus comprising a bias magnetic field generator. Disk-like recording medium 6 (to be referred to as disk 6 hereinafter) is supported, on turntable 8 coupled to motor 7, by clamp member 9, and is rotated at a predetermined speed in accordance with driving of motor 7. In addition, pickup 10 is arranged so as to oppose a recording surface of disk 6. Pickup 10 can be moved by driver 11 along the radial direction of disk 6. Bias magnetic field generator 12 is located above disk 6, whereas pickup 10 is located below disk 6.
In the optomagnetic recording/reproducing apparatus, magnetic fields are generated by units other from bias magnetic field generator 12. For example, a lens actuator in pickup 10, clamp member 9, and driver 11 generate magnetic fields. The lens actuator comprises a coil and a magnet to displace an objective lens for controlling a focusing position of the light beam. For this purpose, the magnetic field is generated by the coil and magnet. A magnet is used for clamp member 9 to support disk 6 by utilizing the attraction force of the magnet, and a magnetic field is generated by clamp member 9. In addition, when a magnetic driver called a VCM (voice coil motor) is used as driver 11, a magnetic field is generated. Thus, leakage magnetic fields are generated by several magnetic field generating elements other from the bias magnetic field generator.
The intensities of the leakage magnetic fields from these magnetic field generating elements have been actually measured. From the results, it was found that the intensity of the magnetic field generated by the lens actuator is about 100 Oe, and the clamp member and the VCM respectively generate the magnetic fields of the order of several tens Oe. Hitherto, these leakage magnetic fields have been considered negligibly low in intensity, as compared with the magnetic field generated by the bias magnetic field generator. However, their total intensity, which amounts to several hundred Oe, is far from negligible. More specifically, unless the leakage magnetic fields, which have been generated by the magnetic field-generating elements and extend in different directions, completely cancel out one another, a leakage magnetic field exists, which inevitably interferes with the bias magnetic field. As a consequence, the bias magnetic field is attenuated in the direction perpendicular to the recording surface of medium 6, and becomes too weak to generate a sufficiently strong magnetic field for recording data on recording medium 6. In order to obtain a desired magnetic field, a larger current may be supplied to a coil for generating the bias magnetic field. In this case, however, the coil is heated, thus posing another problem. In addition, the number of turns may be increased to generate a required bias magnetic field. In this case, however, the size of the unit is increased.